Kelp
Kelp is a very common ingredient in Chinese and Japanese dishes. It has green and thick leaves, very hard to bite (from a kid's perspective) and very salty (because it's from the sea).
I was not a big fan for Kelp when I was little, but my mom always made me eat them, saying that it was an important source of Iodine. Nowadays, as iodized salt becomes almost popular in China, I no longer need to eat it just for the sake of getting Iodine.
However, I actually like to get it once in a while, and enjoy the "chewiness". I don't know where you can get them from local markets like Safeway or Trader Joe's, but you should be able to find them from a small market in the China Town.
